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Trans-lunar injection
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<p>A trans-lunar injection (TLI) is a <a href="/facts/Orbital_maneuver/HAlEFRSJ">propulsive maneuver</a>, which is used to send a <a href="/facts/Spacecraft/YQqQaUw0">spacecraft</a> to the <a href="/facts/Moon/zKHqEMPb">Moon</a>. Typical lunar transfer trajectories approximate <a href="/facts/Hohmann_transfer_orbit/qjhbKTLX">Hohmann transfers</a>, although <a href="/facts/Low-energy_transfer/UTQPfCFN">low-energy transfers</a> have also been used in some cases, as with the <a href="/facts/Hiten_(spacecraft)/koCLav2J">Hiten</a> probe. For short duration missions without significant <a href="/facts/Perturbation_theory/JWrNgbVm">perturbations</a> from sources outside the Earth-Moon system, a fast Hohmann transfer is typically more practical. </p><p>A spacecraft performs TLI to begin a lunar transfer from a low circular <a href="/facts/Parking_orbit/wAk1jTxe">parking orbit</a> around <a href="/facts/Earth/HLLmDfj0">Earth</a>. The large TLI <a href="/facts/Orbital_maneuver/HAlEFRSJ">burn</a>, usually performed by a chemical <a href="/facts/Rocket/b6j4keIl">rocket</a> engine, increases the spacecraft's velocity, changing its orbit from a circular <a href="/facts/Low_Earth_orbit/rq1TwayJ">low Earth orbit</a> to a highly <a href="/facts/Elliptical_orbit/SfuXuATN">eccentric orbit</a>. As the spacecraft begins coasting on the lunar transfer arc, its trajectory approximates an elliptical orbit about the Earth with an <a href="/facts/Apsis/VaL6ow9w">apogee</a> near to the radius of the Moon's orbit. The TLI burn is sized and timed to precisely target the Moon as it revolves around the Earth. The burn is timed so that the spacecraft nears apogee as the Moon approaches. Finally, the spacecraft enters the Moon's <a href="/facts/Sphere_of_influence_(astrodynamics)/L3r5zYMo">sphere of influence</a>, making a hyperbolic lunar swingby. </p>